This invention relates to a cutter for sheet metal and is more specifically concerned with a portable sheet metal cutter.
A portable sheet metal cutter may use the scissors principle or the guillotine principle. These principles of operation are described in detail in Australian patent specification No. 49614/69 entitled "AN IMPROVED TIN-SNIPS, A HAND GUILLOTINE FOR CUTTING SHEET METAL". Briefly, the scissors principle relies on two moving blades operated from opposite sides of the cut whereas the guillotine principle relies on a fixed blade and a moving blade both disposed on the ends of arms which extend away from the same side of the cut.
From a practical point of view the guillotine principle is preferable, because the metal portions severed from one another by the cut may be of some length and can easily obstruct the operator's hands, if the cutter is using the scissors principle. Also, if the cutter is to be motor driven a more compact assembly results if the blade-support arms extend away from the cut alongside one another.
A motor-driven portable sheet metal cutter has been proposed in the U.S. Pat. No. 4,173,069 using three blades. The centre blade is reciprocated back-and-forth and is sandwiched between two fixed blades. Each fixed blade has one shearing face and the movable blade has two shearing faces--co-operating with the respective fixed blade faces to provide two parallel cuts. A problem with such an arrangement is that thin sheet metal tends to flex in the vicinity of the cut and makes it hard for the operator to follow a scribed line or other marker defining the line of the cut. Also the moving blade shearing surface is disposed beneath the cut and the two cuts, one for each fixed blade, are spaced by the thickness of the moving blade and result in the sheet being severed into three pieces.
A simpler form of portable sheet metal cutter is described in U.S. Pat. No. 2,635,335. The cutter is provided at one end of a head adapted to be attached to a portable electric drill. The head carries a pair of blades pivoted to one another and one of which is movable and the other fixed. The movable blade is oscillated about the pivot axis by an extension arm which is formed at its end with a follower riding in an endless groove guide surface eccentrically arranged around the axis of symmetry of a rotatable disc. The disc is carried inside the head by a drive shaft which passes through spaced bearings inside the head and has its other end formed as a stub-shaft which can be gripped by a chuck of the drill.
The fixed blade is in the form of an L-shaped plate having one limb forming a flat leg attached to the head, and the other limb forming a foot extending laterally away from the free end of the leg on one side so as to lie in a plane perpendicular to the leg. The edge of the foot adjacent the leg is provided with a linear edge spaced from the plane of the leg and formed with a shearing face lying in a plane perpendicular to the plane containing the axis of the pivot.
The movable blade is formed as a flat plate lying against the leg of the fixed blade and terminating at its lower end in a sharp straight corner which lies in substantially the same plane as the shearing face of the fixed blade. The end-portion of the moving blade is accomodated in the gap between the fixed shearing face and the plane of the leg so that the portion of the moving blade between the pivot and its lower corner edge is in compression when a sheet of metal is being sheared, to transmit the compressive force of the cut through the metal of the flat movable blade to the pivot.
The cutter described is effective for cutting along straight lines or for following curves in one direction, but it cannot be used to follow curves in the opposite direction because of the arrangement of the parts which causes the edge of the workpiece above the foot to butt against the exposed face of the movable blade and prevent the cutter being turned in the direction opposite to that of the foot. To cut along curves in said other direction, it is necessary to detach the head from the drill and fit a different head having the relative positions of the fixed and moving blades interchanged.